Page 12: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Carbon Tetrachloride

Part II. Science and Technical Considerations - Continued

11.0 Rationale

Although carbon tetrachloride is no longer produced in Canada, Canadians may be exposed to carbon tetrachloride through air and drinking water due to its continued presence in the environment. Carbon tetrachloride is unlikely to pose a concern from surface water sources because of its high volatility, but may be a problem from groundwater. Carbon tetrachloride's high volatility indicates that inhalation and dermal absorption during bathing and showering may also serve as important routes of exposure. As a result, this assessment incorporates a multi-route exposure approach.

Carbon tetrachloride is classified as a possible human carcinogen, based on inadequate evidence of carcinogenicity in humans, but sufficient evidence in animals. This is consistent with the classifications established by both IARC and the U.S. EPA. A TDI approach was chosen to derive the Maximum Acceptable Concentration for carbon tetrachloride in drinking water, because of the major deficiencies in the available cancer studies, and because animal studies suggest that the carcinogenicity of carbon tetrachloride is secondary to its hepatotoxic effects, indicating a possible threshold. Consequently, the MAC of 0.002 mg/L is established based on hepatotoxicity (observed increase in SDH activity and mild hepatic centrilobular vacuolization in a subchronic rat study), and incorporates an additional uncertainty factor of 10 to accommodate for database deficiencies.

This MAC is achievable by available treatment technology at both the municipal and residential scales, and measurable by available analytical methods. As part of its ongoing guideline review process, Health Canada will continue to monitor new research in this area and recommend any change to the guideline that it deems necessary.

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